This invention relates to light exposure apparatus, and more particularly to light exposure apparatus for forming a fluorescent screen of a colour picture tube.
The colour picture tube is usually constructed such that a fluorescent screen which is formed on the inner surface of the panel of the tube by arranging a plurality of phosphors of red, blue and green colours in the form of dots or stripes is selectively excited through a colour selection electrode by means of electron beams emanated from an electron gun assembly, whereby to reproduce picture images. The fluorescent screen is usually formed as follows. For example, the light emitted from a light source is transmitted through a correction lens which is used for the purpose of correcting the apparent position of the light source to expose to the light the fluorescent screen on the inner surface of the panel after passing through a colour selection electrode (such as a shadow mask) which is disposed at a predetermined distance from the correction lens and with a predetermined relation with respect to the fluorescent screen to be exposed. As a result, the photosensitive material coated on the exposed portion of the fluorescent screen is exposed to form the fluorescent screen having a desired arrangement of dots or stripes.
The dimension of the dots, or the width of the stripes formed on the fluorescent screen in this manner is determined in accordance with the quantity of the exposure light which in turn is determined by the position of the dots or stripes on the fluorescent screen. Usually, for the purpose of obtaining a light quantity distribution necessary to subject the positions on the fluorescent screen on which the dots or stripes are to be formed to the exposure of the light having an adequate value quantity, an illumination intensity correcting filter having a proper density distribution with respect to the exposure light is disposed between the light source and the colour selection electrode.
Although the illumination intensity correcting filter having a proper illumination density distribution is generally prepared by vacuum deposition technique or photographic process it is rather difficult to obtain the desired density distribution.
Especially, it is very difficult to finely adjust the density of the filter at a local area thereof.
More particularly, since the illumination intensity distribution desired on the fluorescent screen is not always symmetrical, the density distribution of the illumination intensity correcting filter would also be asymmetrical if it were made to correspond to the desired illumination intensity distribution, so that if such correction filter were to be prepared by the conventional vacuum deposition technique or photographic process, the number of the process steps would be greatly increased. Yet a satisfactory filter having the desired density distribution cannot be obtained in spite of a large increase in the number of the process steps. Accordingly, fine adjustment of the density of the filter at a particular local area thereof is almost impossible and hence the illumination intensity correction of the exposure light on the fluorescent screen by means of an illumination intensity correcting filter has been deemed to be almost impossible.
Recently, colour picture tubes of the so-called black matrix type wherein the areas of the fluorescent screen except the portion of the phosphor dots or stripes are coated with graphite have been offered. In the tubes of this type it is of utmost importance to make the dimension of the phosphor dots or stripes as uniform as possible for the purpose of decreasing non-uniform white because the phosphor dots or stripes as a whole contribute to luminescence. It is necessary to correct the exposure light quantity distribution in order to maintain the dimension of the phosphor dots or stripes at a definite value. For this reason, it is necessary to more accurately control the density distribution of the filter than the prior art practice.